Gauging heads are known—more specifically gauging heads for checking dimensions of pieces being machined in grinding machines—which include one or two movable arms, each comprising two mutually adjustable portions, a first portion coupled with a transducer of the head and a second portion carrying a feeler for contacting the pieces to be checked.
Italian patent No. 1179306 describes some forms of gauging heads of this type, in which the two portions of the movable arm are mutually coupled by means of frictional coupling, while zero-setting devices with movable mechanical references, located at the exterior of the casing of their associated heads, enable to accomplish, in an extremely simple way, operations for mechanically zero-setting the heads. More specifically, the mechanical references can be arranged in a zero-setting position that determines and sets, or contributes to set, the arrangement of one of the two portions of the movable arm, the one that is directly coupled with the transducer. This arrangement corresponds to a zero configuration of the transducer and is locked when it is required to change from the checking of pieces with a determined nominal dimension, for example a determined diameter, to the checking of pieces with a different nominal dimension (diameter). In order to “zero-set” the head, a master piece with the fresh nominal dimension is positioned in the measuring position (for example between the live center and the dead center of an external grinding machine) and a force is manually applied to the portion of the arm carrying the feeler. This force is sufficiently strong to alter the arrangement with respect to the other portion and to displace the feeler to contact the master piece. By virtue of the presence of the mechanical references the transducer signal remains, in the course of this operation, roughly at a zero measurement value.
Once the feeler is displaced to contact the piece and the force manually applied to the arm is removed, the mechanical zero-setting operation ends. The mechanical references are moved for unlocking the portion of the arm coupled with the transducer and for enabling measurement displacements of the entire arm, while the arrangement of the two portions that form it remains unaltered thanks to the frictional coupling.
The zero-setting is more accurately defined by carrying out a simple electric zero-setting operation that consists in operating a potentiometer located in an amplifier which detects and displays the signal of the transducer/s associated with the movable arm/s.
Thus, in the heads according to Italian patent No. 1179306 it is possible to carry out, in an extremely simple and rapid way, mechanical zero-setting operations thanks to the locking of the zero position. In this way lengthy and repeated operations, that experienced and skilled operators have to perform in order to reach the condition whereby the feeler/s contacts/contact the master piece and, concurrently, a measurement signal is approximately at zero value, can be avoided. By virtue of the fact that both the frictional coupling and the movable mechanical references are at the exterior of the head casing, it is furthermore possible to apply the zero-setting mechanism to conventional heads without any need of considerable constructional changes and/or constructional changes which may affect the component parts of the head arranged within the casing.
In the course of the operations for checking mechanical pieces machined in a machine tool or in the approach towards/displace away phases between head and piece, the arms may undergo impacts. Even though such impacts may be small and insufficient to damage the arms or other component parts of the head, they may alter the arrangement, set by the frictional coupling, between the portions of the arm and consequently alter the zero position. In addition to the serious problems caused by the need to interrupt the machining in order to repeat the zero-setting operations, there may occur situations in which the altering of the zero position is not noticed and the checking operations are not stopped and consequent errors could arise in the course of the machining of the pieces.
Furthermore in the heads according to previously mentioned Italian patent, the correct zero position could be altered in an unpredictable way by arm flexures caused by the not negligible force that it is required to apply, in the course of the zero-setting operations, to alter the frictional coupling between the portions of each arm.